1. Field of the Invention
The present invention relates to a polycrystalline silicon thin film for display devices, a fabrication method thereof, and a thin film transistor fabricated using the same. The present invention relates more particularly to a polycrystalline silicon thin film fabricated by controlling the shape of the silicon grains, a fabrication method of the thin film, and a thin film transistor fabricated using the polycrystalline silicon thin film.
2. Description of the Related Art
Ordinarily, the sequential lateral solidification (SLS) method is used for crystallizing the silicon grains by overlappingly irradiating a laser beam onto an amorphous silicon layer two or more times so that silicon grains are laterally grown. Polycrystalline silicon grains manufactured using the SLS method are characterized in that they are formed in a cylindrical shape lengthy from end to end, and grain boundaries are generated between adjacent grains due to the limited size of the grains.
It is reported that polycrystalline or single crystalline grains are capable of forming large silicon grains on a substrate using the SLS crystallization technology, and a thin film transistor (TFT) fabricated using the large silicon grains is capable of obtaining characteristics similar to characteristics of a TFT fabricated using single crystalline silicon.
FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D represent an ordinary SLS crystallization method.
In the SLS crystallization method, amorphous silicon is melted in the laser beam transmission region if a laser beam is irradiated onto an amorphous silicon thin film layer through a mask having a laser beam transmission region and a laser beam non-transmission region as shown in FIG. 1A.
Crystallization occurs at an amorphous silicon/molten silicon interface when cooling begins after irradiation of the laser beam is completed, wherein a temperature gradient is formed in such a way that the temperature of the silicon is gradually reduced from the amorphous silicon/molten silicon interface toward the central part of the molten silicon layer, the molten silicon solidifying, and crystallizing, as the heat dissipates.
Therefore, referring to FIG. 1B, a polycrystalline silicon thin film layer, having grains that are formed in a long cylindrical shape, is formed. Polycrystalline silicon grains are laterally grown until the molten silicon layer is completely solidified as heat flux flows from the mask interface to the central part of the molten silicon layer.
As illustrated in FIG. 1C and FIG. 1D, by moving the laser beam transmission region of the mask so that it exposes more of the amorphous silicon and a portion of the crystalline silicon, and irradiating this exposed area with the laser beam, the length of the grains is increased, with silicon atoms being adhered to already formed polycrystalline silicon grains that are not melted, due to being covered by the mask, as the partially melted amorphous silicon thin film and crystallized silicon layer are being cooled thereafter.
Therefore, TFT characteristics close to single crystalline silicon can be obtained, since a barrier effect of a grain boundary to a charge carrier direction is minimized in the case that an active channel direction is parallel to the direction of the grains grown by the SLS method when fabricating the TFT. TFT characteristics are greatly deteriorated in the case that the active channel direction is perpendicular to a grain growing direction, because a plurality of grain boundaries act as a trap of the charge carriers.
The mounting possibility of a circuit is restricted, because the TFT characteristics are greatly changed depending on the active channel direction in the case of the TFT being fabricated by an existing SLS method.
On the other hand, it is disclosed in PCT International Publication No. WO 97/45827 and U.S. Pat. No. 6,322,625 that amorphous silicon on the whole substrate is converted into polycrystalline silicon, or only a selected region on the substrate is crystallized by SLS technology after depositing the amorphous silicon on a substrate.
Furthermore, when fabricating TFTs for a liquid crystal display (LCD) device, comprising a driver and pixel array, by forming large silicon grains using the SLS crystallization technology, characteristics of TFTs similar to characteristics of TFTs fabricated using single crystalline silicon can be obtained because the barrier effect of grain boundaries for a charge carrier direction is minimized in the case that an active channel direction is parallel to the direction of the grains grown by the SLS crystallization method, as described in U.S. Pat. No. 6,177,391. But a plurality of grain boundaries which act as traps of the charge carrier exist in this method, and also TFT characteristics are greatly deteriorated in the case that the active channel direction is perpendicular to the growing direction of the grains in patents like this.
Actually, there are cases in which TFTs inside a driving circuit are generally perpendicular to the TFT in a pixel cell region when fabricating an active matrix display, wherein the uniformity of the device can be improved by fabricating the active matrix display in such a way that the direction of the active channel regions is inclined to the crystal growing direction at an angle of 30° to 60° in order to improve the uniformity of characteristics between TFTs, while the characteristics of each TFT are not greatly deteriorated.
However, it is likely that fatal grain boundaries are included in the active channel regions, since this method also uses grains having a limited size formed by the SLS crystallization technology. Therefore, there is a problem in that unpredictable non-uniformity causes a difference of characteristics between TFTs in this method.